The attachment of surgical needles to surgical sutures is a mechanical problem which is most challenging. If an eye is used, so that the thread is doubled through the eye of a needle, as in conventional sewing, the doubled end of the suture in the eye must pass through tissues during use, which enlarges the suture hole in tissue leading towards loss of liquid tightness, and increased trauma.
There has long been a trend towards eyeless needles in which the end of the suture is fixed to the needle so that the suture is pulled axially through the tissue causing a minimum of trauma. Preferably, it is desirable that the needle have a diameter that is only slightly larger than the suture. The attachment must be one which (a) is predictably secure, (b) causes a minimum of damage to tissue, (c) is convenient for the using surgeon, (d) permits sterilization and (e) entails reasonable costs, so that the suture needle may be attached economically. The attachment must stand up under the rigors of manufacture, sterilization, storage and shipment and use.
Rethreadable needles have many theoretical advantages, such as permitting reuse of the needle to avoid the cost of a new needle for each suture and because of such reuse can have a more expensive quality of steel and a more expensive quality of point than a disposable needle. However, any reusable needle requires some system for attaching the suture to the needle which permits reloading of a suture in the needle either in the operating room by operating personnel under sterile conditions or outside of the operating room under conditions which permits cleaning and sterilization of the needle, coordinated with the attachment of the suture, under such conditions that the final product is clean, sterile, and readily stored between the time of reloading and the time of use. With requirements for sterility, low cost, convenience and a minimum of size of the attachment system to avoid an unduly large surgical needle hole in tissue and a desirably smooth exterior so that the needle will slide through the tissue with a minimum of damage, so many conflicting requirements are presented that reloadable low trauma needles are not in common use.
The most common type of surgical needle is a single use needle of appropriate size and shape which is crimped to the end of the suture, so that the needle is used but once and then discarded. The attachment can be either a "drilled end" needle, that is one in which a concentric hole is formed in the end of the needle in which the suture is placed and the needle crimped around the suture; or a "flange" needle in which a U-shaped channel is stamped into the end of the needle with the ends of the U being crimped about the suture to hold the suture.
In either system, the crimp must be strong enough so that the suture remains attached to the needle during the placement of the needle and the attached suture through tissues. After the suture is placed in the tissues, it is frequently convenient to detach the needle so that at an appropriate time in the surgical procedure the ends of the suture can be tied together without the needle dangling around to stick or cut the patient, surgeon or nurse. Cutting of the suture with scissors or a scapel is a convenient method of disengaging the needle but requires an extra instrument and an extra manipulation. It would seem particularly convenient to pull the needle from the suture if the attachment of the needle to the suture is such that convenient disengagement may occur. Some surgeons have used such techniques with standard needles having pull-off values in conformity with U.S. Pharmacopeia requirements.
The pull required for pulling the needle off the suture or breaking the suture is referred to in the U.S. Pharmacopeia as needle attachment, or testing the security of attachment of eyeless needles to sutures. For convenience the term pull-off is herein used, as the present invention is to permit the control of pull-off to desired limits--often well below the breaking strength of the suture on either straight pull or knot pull.
Needles have been suggested in which from either a cutting edge in the needle, or the firmness of attachment, the needle can be detached from the suture at a reasonably predictable value which is acceptable.
The problem is that the pull-off must be at a sufficiently high value that the suture may be placed without risk of the needle becoming detached from the suture during placement; and yet it must pull-off at a value sufficiently far below the breaking strength of the suture that the needle will predictably pull-off before the suture breaks, and additionally must pull-off at a value which is reasonably exertable upon the needle by the surgeon at the time of use.
These theoretical objectives are difficult to obtain in practice. Manufacturing tolerances for the needle and the suture are such that a predictable acceptable pull-off value has been difficult to achieve under commercially practical manufacturing practices, so that firm competent attachment at U.S. Pharmacopeia standards has been usual.
For instance, whereas the wire size from which needles are made may vary in the order of a thousandths of an inch between lots, it is difficult to hold the outside diameter of the needle to less than plus or minus about two tenthousandths in a single lot. It is difficult in production to drill or otherwise form a hole with less than a variation of plus or minus another two tenthousandths of an inch.
The braid size of a suture always varies somewhat. Additionally, to aid in insertion in the needles it is conventional practice to end-dip. Sutures stock is wrapped on a frame under a comparatively uniform starting tension. However, the frame distorts from the large numbers of turns of suture strand under tension placed on the frame, and as the frame is strained, the tension on the initial turns of suture tends to be reduced.
Additionally variations in the coating material, the time and concentration of the bath and drying conditions are such that the end-dipped size may vary in the order of about a thousandth of an inch within a single production run.
Conventional crimp operations are difficult to control. Usually crimp is between dies that close to a fixed gap, and any variation in the crimp dies, the needle size, the hole size and the suture size changes the degree of crimp, and the change can be larger than is acceptable in trying to manufacture a controlled release or controlled pull-off suture-needle combination. Quality control is difficult.
Other approaches such as crimping to a controlled pressure, or using a combined pressure and gap cycle have been considered. The usual approach is to increase the degree of crimp until the crimping shears off the suture on a test sample, then back off on the degree of crimp until the sutures are not sheared off, and pull-off above USP limits, and after testing a representative group of sutures, proceeding to production.
In many instances each suture is non-destructively tested to a minimum pull-off value in routine production. Only those that pass a test load are accepted for further processing.
In addition to size effects, the surface smoothness of the suture and the needle hole, and lubricants on either affect the pull-off values. Some needles are lubricated with a silicone or polytetrafluoroethylene to reduce tissue drag in use. The needle hole can be filled with xylene or other volatile solvent during coating to reduce the penetration of the lubricant into the needle-hole.
In theory, if all process parameters are perfectly controlled, replicate results are obtained. In practice, so many factors enter that adequate control in production is easier to describe than accomplish.